Generally, research on secondary batteries that can be discharged and recharged, unlike primary batteries, has been actively carried out along with the development of advanced technologies, such as digital cameras, cellular phones, notebook computers, or hybrid vehicles. Secondary batteries may include a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery, a lithium ion secondary battery, etc. Of these batteries, the lithium ion secondary battery has an operating voltage of 3.6V or more, and is used for power for portable electronic devices, or is used for a high power hybrid vehicle in such a way that several batteries or several tens of batteries are connected in series with each other. Since such a lithium ion secondary battery has an operating voltage that is three times as high as that of the nickel-cadmium battery or nickel-metal hydride battery, and has excellent characteristics of energy density per unit weight, the popularity of the lithium ion secondary battery has rapidly increased.
A lithium ion secondary battery can be manufactured in various shapes. Representative shapes thereof include a cylinder type and a prismatic type, which are mainly used for lithium ion batteries. A lithium polymer battery that has recently been popularized is manufactured as a flexible pouch-type battery, so that the shape thereof can be relatively freely implemented. Further, since a lithium polymer battery is very safe and is also lightweight, such a lithium polymer battery is considered suitable for the realization of portable electronic devices having slim and lightweight structures.
Referring to FIG. 1, a conventional pouch-type lithium ion secondary battery 10 includes a battery part 11, and a casing 12 for providing an inner space 12a in which the battery part 11 is accommodated.
The battery part 11 has a shape in which a positive electrode plate 11a, a separator 11c, and a negative electrode plate 11b are stacked together. Respective plates of the battery part 11 are electrically connected to positive and negative electrode tabs 13 and 14.
First ends of the positive and negative electrode plates 11a and 11b protrude outward from the casing 12 through the sealing portions 12b of the casing 12. The protruding ends of the positive and negative electrode tabs 13 and 14 are connected to the terminals of a protection circuit board (not shown).
Pieces of sealing tape 15 are respectively wound around the external surfaces of the positive and negative electrode tabs 13 and 14 so that an electrical short circuit between the casing 12 and the electrode tabs 13 and 14 can be prevented at portions that come into contact with the sealing portions 12b. 
Such a casing 12 is a pouch-type casing that includes a middle layer made of a metallic foil, and inner and outer surface layers which are attached to both sides of the metallic foil and are made of insulating films, unlike the structure of a cylinder type or prismatic type can, which is molded using a thick metallic film. The pouch-type casing can be freely bent thanks to the excellent molding characteristics thereof. In such a pouch-type casing 12, the inner space 12a capable of accommodating the battery part 11 is formed, as described above, and the sealing portions 12b provided to the thermal adhesion surface are formed along the border of the inner space 12a. 
FIG. 2 is an enlarged view taken along line II-II of FIG. 1.
The casing 12 is a composite film composed of a middle layer, which is made of metallic foil, for example, aluminum foil, and inner and outer surface layers which are attached to the inner and outer sides of the middle layer and are made of insulating films in order to protect the middle layer.
In the inner space 12a formed in the casing 12, the battery part 11, in which the positive electrode plate 11a, the separator 11c, and the negative electrode plate 11b are sequentially arranged, is accommodated. The positive electrode tab 13 and the negative electrode tab 14 are drawn from the positive electrode plate 11a and the negative electrode plate 11b, respectively, as shown in FIG. 1. The ends of the drawn electrode tabs 13 and 14 are exposed to the outside through the sealing portions 12b of the casing 12. In the sealing portions 12b, the outer surfaces of the electrode tabs 13 and 14 are covered with sealing tape 15.
The pouch-type lithium ion secondary battery 10 having the above structure is constructed so that the battery part 11 is completed by electrically connecting the positive and negative electrode tabs 13 and 14 to the positive and negative electrode plates 11a and 11b, respectively, and by winding the positive electrode plate 11a, the separator 11c, and the negative electrode plate 11b in any one direction after sequentially arranging them.
The completed battery part 11 is mounted in the casing 12, having the inner space 12a therein, through a drawing process, and the first ends of respective electrode tabs 13 and 14 are exposed to the outside of the casing 12 when the battery part 11 is mounted. In this state, predetermined heat and pressure are applied to the sealing portions 12b of the casing 12 to conduct thermal adhesion, and thus the pouch-type lithium ion secondary battery 10 is completed.
In order to stabilize the structure of the pouch-type lithium ion secondary battery 10, the presence of abnormalities in the battery is determined through a series of formation processes, such as charging, aging, and discharging.
Meanwhile, when a high power lithium battery is required, as in the case of a hybrid vehicle, several tens to several hundreds of pouches having the same structure as the pouch of FIGS. 1 and 2 are stacked together and are connected in series with each other, so that a high voltage can be obtained.
Since such a pouch-type lithium polymer battery is implemented using a weak aluminum pouch that can be easily bent or curved, it can be used for a long period of time only when the aluminum pouch is protected using a robust casing unit. In consideration of the fact, the present applicant proposed a preferred embodiment in Korean Patent Appln. No. 2005-24172.
The technical details of the above patent are described below. As shown in FIG. 3, a unit cell 31 is constructed to include a pouch 34 having a lithium ion secondary battery therein, and a pair of positive electrode tab 32 and negative electrode tab 33 formed on any one surface of the pouch 34 in the shape of a shelf so that the ends of respective electrode tabs 32 and 33 are bent in opposite directions.
As shown in FIGS. 4 and 5, the lithium battery unit cell 31 having such electrode tabs 32 and 33 includes separate frame members to maintain the states of the parallel and vertically standing electrode tabs, to decrease the temperature of each lithium battery unit cell 31, which radiates heat, and to realize smooth heat radiation. Such frame members include a main frame 41 having lattice-shaped paths 41b in the inner space capable of accommodating the lithium battery unit cell 31, and a partitioning frame 42 functioning as a partition wall between respective main frames 41 and having lattice-shaped paths 42b corresponding to the lattice-shaped paths 41b, and have a partitioned structure in which the frames are laterally and alternately arranged in parallel with each other.
Further, a heat radiation part 41a having a thin film shape which is much smaller than the width of the inner space for accommodating the lithium battery unit cell 31 is formed on the upper portion of the main frame 41. Each cooling channel 43 having a structure allowing air to be blown is formed to correspond to the radiation part 41a of the main frame 41, implemented through the stacked combination (partitioned arrangement) of the main frame 41 with the partitioning frame 42.
FIGS. 6 and 7 are views showing the structure in which a cover 51 encloses the stacked combination structure of the main frame 41 and the partitioning frame 42, and in which a cooling fan 52 is installed offset from the center of a inlet 53, and air blown by the cooling fan 52 cools the accommodated lithium battery unit cell 31 while passing through the lattice-shaped paths 41b and 42b via the heat radiation part 41a of the main frame 41 and the partitioning frame 42, that is, the cooling channel 43, within the space partitioned by the cover 51, and is then exhausted through an outlet 54 formed on the rear part of the cover 51.
However, since air blown by the cooling fan passes by the lattice-shaped paths through the cooling channel formed between the upper portions of the main frame and the partitioning frame, the line of flow of the blown air is long, and thus the heat radiation efficiency of the lithium battery unit cell is not actually high.